The simpliest way is to install the file manage from here, go to directory with screen savers and delete all screen saver pictures you don't like.

What happens if we remove all the files, does it show the last page
in standby mode ? Is there any indication then that it is in standby ?

Also the feature for configurable standby and shutdown time has been
pending for quite sometime. IMHO this is a basic feature of an ereader which
directly affects the functionality the ereader is intended for.